Method of manufacturing billets from powder

ABSTRACT

In the manufacture of billets from metal powder, in which the powder is enclosed in a container and subjected to isostatic pressure in a pressure furnace at such a temperature as to solidify the powder body, the container after the powder is introduced thereinto is degassed and thereafter supplied with a gas which is harmless to the powder material, which gas is present in the subsequent heating as a heat transfer agent circulating in the container.

United States Patent Bergman et al. July 8, 1975 [54] METHOD OF MANUFACTURING BILLETS 3,384,481 5/1968 Broverman 75/226 3,469,976 9/1969 ller 75/226 FROM POWDER 3,698,962 l0/l972 Kasak et a]v 75/226 [75] Inventors: Carl Bergman; Nils I ar L n gr n; 3,700,435 10/1972 Chandhok 75/226 Hans Gunnar Larsson, all of Vasteras; Torsten Strandberg, SOdeFfOI'S, all Of Sweden Primary ExaminerBenjamin R. Padgett [73] Assignee: Allmanna Svenska Elektriska Assistant Examiner-B Hum Aktiebolaget, Vasteras, Sweden [22] Filed: June 5, 1973 21 App]. No.: 367,105 [57] ABSTRACT [n the manufacture of billets from metal powder, in 8" Apphcatlon Prlomy Data which the powder is enclosed in a container and sub- June 12, 1972 Sweden 7685/72 jected to isostatic pressure in a pressure furnace at such a temperature as to solidify the powder body, the [52] U.S. Cl 75/224; 75/226 container after the powder is introduced thereinto is [5|] Int. Cl B22f 1/00 degassed and thereafter supplied with a gas which is [58] Field of Search 75/226, 223, 224 harmless to the powder material, which gas is present in the subsequent heating as a heat transfer agent cir- [56] References Cited culating in the container.

UNITED STATES PATENTS 3,341,325 9/1967 Cloran 75/226 1 Claim, 5 Drawing Figures METHOD OF MANUFACTURING BILLETS FROM POWDER BACKGROUND OF THE INVENTION l. Field of the Invention The present invention relates to a method of manufacturing billets to be further worked to the desired shape by means of rolling, forging or machining with a power as the starting material.

2. The Prior Art Conventional metallurgical manufacture by smelting of alloys which have a high tendency to segregate, for example high-speed steel, involves very difficult problems. In the cast state these materials have a very coarse structure and the chemical composition may vary between different parts of an ingot. As a consequence, plastic machining by means of, for example, rolling or forging is extremely difficult to carry out. Certain alloys cannot be machined plastically at all. Furthermore, many materials still have a relatively coarse and uneven structure even after the plastic machining. This has limited the possibilities of manufacturing on an industrial scale articles made from materials having certain compositions by conventional metallurgical smelting manufacture. Experiments have therefore been made starting with a powder to manufacture billets to replace ingots, by enclosing the pow der in a casing and sintering it under high pressure so that a homogeneous, solid body is obtained. The experiments have succeeded and high-speed steel, for example, has been successfully manufactured of a composition which could previously be manufactured only with great difficulty and normally only to a very limited extent by conventional methods available. When rolling or forging billets manufactured from powder, products of uniform and high quality have been obtained, thanks to the homogeneity and fine-grained structure of the starting material. However, the methods of production so far have only permitted production on a limited industrial scale.

According to one method, a billet shaped from powder is enclosed in a container and is isostatically compressed by subjecting the container with the enclosed powder to a high, all-sided external pressure in a pressure chamber. This compression is carried out at room temperature. After the compression, the container is inserted into a heating furnace and an evacuation opening is connected to a vacuum pump. The container with the powder is heated while being simultaneously degassed, after which the evacuation opening is closed. Finally the container is inserted in a pressure furnace and is subjected simultaneously to high pressure and high temperature so that the powder is sintered and further compressed.

A billet can be shaped in the normal manner by compression and then be sealed within a steel container of sheet metal. For large cylindrical billets, for example to replace ingots for rolling, the container may be used as a mould during the manufacture. Powder is poured directly into a cylindrical container without a lid and is packed in this container during the filling process, after which it is closed with a lid which is connected in a gastight manner to the cylindrical part of the container. From the technical point of view, the shape of a con tainer is of no importance per se. However, a cylindrical shape best exploits the space of the expensive pressure sintering furnace and thus gives the lowest volume cost. It is possible, however, by this method to manufacture billets of very complicated shape, for example disc-shaped billets for side-milling cutters with projecting teeth, which can only be given the exact size and shape by machining.

The container is preferably heated in two stages. The heating processes may be performed in one and the same furnace, but it may be practical to perform them in two stages in two separate furnaces.

A fine-grained powder has a considerable surface area in relation to the volume ofmaterial and thus great affinity to surrounding gases. Gases may partly be ab sorbed on the surface, partly form compounds with the material included in the powder, partly be dissolved in the powder material. Particularly damaging substances are oxygen, (0 and hydrogen (H The powder is therefore manufactured in an inert gas atmosphere. It is often stored and handled in an inert gas atmosphere as well. Argon is a suitable protective gas.

it has been proved that the density of the powder material influences the heating. it has proved valuable to compress a billet before heating, thus increasing its density. The compression should take place at a pressure of at least 1000 bar and at low temperature. Temperatures between 0 and 300C are applicable. As a rule the compression can be performed at room temperature. For material of the highspeed steel type with powder grains of spherical type, a pressure of about 4000 bar has given a good result. It has been found that, within a certain pressure range, a sudden increase in the thermal conductivity occurs so that, if compression is carried out at a pressure above this level, it is possible to heat a billet to sintering temperature in less than half the time it takes to heat a billet which has been compressed at a comparatively negligibiy lower pressure, but which lies below said limit.

SUMMARY OF THE INVENTION By means of the present invention the time for heating a powder-filled container is considerably reduced, because the container, for at least part of the heating process, contains a gas which is harmless in relation to the powder material, said gas acting as a heat transfer agent. It has been found that the porosity of the powder mass in the container is sufficiently great to obtain a considerable gas circulation between warm and cold parts in the powder, so that heat is quickly transferred to the cold parts. In this way, a considerable heat trans fer from the powder in the exterior part of the con- 0 tainer to the powder in its interior part is obtained during heating. The heating time can be reduced by several hours. For very large powder-filled containers the time for heating the container fully through can be reduced by 10 hours and more. A container is suitably compressed in known manner at room temperature. It is provided with an evacuation connection in the form of a spout. The container is evacuated to so low a pressure and for so long a period of timethat the moisture and the gases present in the powder body are removed. This evacuation can in most cases be carried out at room temperature, but a certain heating may be favorable and may expedite the discharge of gas. After this, gas which is hannless to the powder is heated. During heating the container may communicate either with a gas source, containing harmless gas, or it may communicate with a space above the container, for example the furnace space or the space above the furnace. During ieating such a flow of gas out of the container is obained that bursting of the container and flowing of air nto the container can be avoided. By harmless gas is neant a gas having such properties that no harmful, or nsignificant harmful, compounds are formed between he powder material and the gas, a gas having such Jroperties that it forms compounds with harmful sub- ;tances in the powder so that there are removed, or a gas forming favorable compounds with the powder ma- .erial. The harmless gas may be a rare gas, for example helium or argon, or another gas which is neutral in relation to the powder material, for example nitrogen. In some cases it may be appropriate to use a reducing gas together with a rare gas or a neutral gas.

The container may be evacuated before compression, but it is possible to perform the compression satisfactorily while there is still a certain amount of gas left in the container and in spite of this obtain satisfactory density and satisfactory metallurgical properties. This is the case when the gas forms harmless or the desired favorable solid compounds together with the powder material. When pressing steel-based powder, nitrogen gas may be advantageously used since a limited amount of nitrogen may improve the properties of the material. As a rule rare gases must be completely removed.

It is also possible to evacuate the container and add such an adjusted amount of gas that a cold container can be hermetically sealed and heated to hot pressing temperature without the risk of obtaining a harmful overpressure in it. Another possibility is to fill the container with gas and supply it with a pressure-limiting discharge means, serving as a check valve. If the con tainer is heated in a pressure furnace, it is possible to apply a pressure in the container exceeding the atmo spheric pressure. The heat transfer capacity of the enclosed gas will then increase. The container may contain material which absorbs the enclosed gas during hot compression. The container material itself may constitute the absorbing material, but a particular absorbing material can be used.

The sintering temperature depends on the material and to a certain extent on the pressure to which the material is subjected during the sintering. For a material of iron-based high-speed steel containing l.25% C, 4% Cr, 6.3% W, 5% M0, 3.4% V and 8.7% Co, a good result is obtained when the sintering is carried out at a temperature of l [C and at a pressure of I000 bar.

The container enclosing the powder is subjected to high temperature during sintering and an interchange of material may take place between the powder and the material of the container. For alloying substances hav ing high diffusion capacity such as, for example, carbon, a considerable interchange may take place. It is therefore important that the material selected for the container has approximately the same carbon activity at the sintering temperature as the material in the powder enclosed in the container. It has been found that the carbon activity for a container material of steel plate having 010% C, 0.20% Si and 0.35% Mn and a powder material containing 0.85% C, 4.0% Si, 6% W, Mo, 2% Vn and the remainder Fe is approximately the same. When sintering billets at 1 150C and at the pressure 1 kbar, the alterations in the boundary layer for the powder material mentioned have been negligible.

In high pressure furnaces pressure media are required which neither destroy the containers around the powder billets nor attack the construction material in the insulating layer which surround the furnace space proper, the material in the pressure chamber proper or the material in electric resistance elements for heating the furnace. In furnaces for high temperatures above I300C, resistance elements of molybdenum are often used, which are rapidly destroyed upon contact with oxygen gas (0 Inert gases must then be used as the pressure medium. The inert gases helium (He), argon (A) and nitrogen (N are particularly suitable as pressure media.

BRIEF DESCRIPTION OF THE DRAWINGS The method and equipment for carrying out the method are described more closely with reference to the accompanying figures.

FIG. I shows a layout of a production system,

FIG. 2 the upper part of a container filled with powder which is provided with a lid having an evacuation opening,

FIGS. 3 and 4 a press for cold isostatic compression of billets, and

FIG. 5 a pressure sintering furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings 1 designates a storage container for powder and 2 a rotatable table which can be turned stepwise. Close to the table is a store of containers 3. An empty container 3a has been placed on the table which, by turning the table 2 stepwise, is moved between a number of different stations. Opposite the storage container 1 is a container 3b in a filling station where it is filled with a powder from the storage container 1 through a hose or a pipe 64. At the next station is a store oflids 4 with a pipe connection 5 and welding equipment 6. Here a container 3c filled with powder 9 is provided with a lid 4 which is welded to the wall 7 of the container 3 by means of a welding seam 8 as shown in FIG. 2. The container 3d is at a transfer station from which it is transferred to a testing station where the tightness of the containers are checked. There are three containers 3e at this station. The testing equipment is designated 10.

In a press plant II the container with powder is isostatically compressed by being inserted in a pressure chamber and subjected to a high isostatic pressure. The plant 11, which is further described with reference to FIGS. 3 and 4, consists of a high-pressure container 12 supported by a stand 13 and a movable press stand 14 which runs on rails 15. The press stand 14 is of the type having two yokes and two spacers which are held together by a prestressed strip sheath. It is supported by two pairs of brackets 16, in which the shafts 18 of the transport wheels 17 are journalled. An electric motor 19 drives the shaft 18 of one of the wheel pairs by way of a gear drive, not shown, in one of the brackets 16. The transport wheels run on rails IS. The cylinder I2 is provided with two end closures 20 and 21 projecting into the cylinder, the lower one being suspended and vertically movable to a limited extent in the cylinder 12, whereas the upper one can easily be lifted for charging and emptying the container. The container 12 is provided at its ends with flanges 22 and the stand I3 with brackets 23 having holes, through which a rod 24 passes. During charging and emptying the stand is a little distance from the press cylinder, as shown in FIGS.

l and 3. After charging and insertion of the upper end closure 2], the stand is moved with the help of the motor 19 above the cylinder 12 so that their center lines coincide, after which pressure medium is introduced into the pressure chamber. The axial pressure operating on the end closures is taken up by the press stand. After the compression process, the end closures are again moved to their inner positions and the stand is moved away from the cylinder 12, so that the highpressure chamber can once again be opened, emptied and charged with a new billet to be pressed. Beside the press stand 11 there are two finished billets 3f.

After compression the powder-filled containers are heated. The heating can be carried out in two stages. In FIG. 1 a group of pre-heating furnaces 2 are shown. Containers in the furnaces are connected, by way of the pipe connection 5 and a conduit 26 which passes through the lid of the furnace, to a vacuum pump and a gas source with harmless gas 27. When the containers have been evacuated, a desired amount of harmless gas is admitted into the containers. The pressure is suitably kept at the same value as or somewhat higher than the atmospheric pressure, so that air is fully prevented from leaking in. After heating to the desired temperature, the containers are fully or partly evacuated and are transferred to pressure furnaces 32. There may be a certain degree of postheating before cold compression in separate postheating furnaces 30 having auxiliary equipment 31 or in pressure furnaces. The heating furnaces may be of conventional kind, for example electric resistance furnaces.

A group of two pressure furnaces 32a and 32b is shown in FIG. 1. These furnaces are further described with reference to FIG. 5. The furnaces are of the type described in US. Pat. No. 3,695,597 and are thus charged from below. The furnaces comprise a furnace room which is enclosed in a pressure chamber. This pressure chamber consists of a high-pressure cylinder 33 of the type which is built up ofa tube 34 and a sur rounding prestressed strip sheath 35, an upper end closure 36 and a lower end closure 37. The cylinder is suspended in a stand 38. The upper end closure 36 remains permanently in the cylinder and is designed with a channel 38 for the supply of pressure medium and a channel for an electric cable 40 to feed electric heating elements 41 and to obtain measuring values from the thermoelement. Above the end closure 36 lies a plate 42 with an outlet for the cable 40. In the upper end closure 36 are an insulating sheath 43 and an insulating lid 44 which separate the actual furnace space 45 from the inner wall of the tube 34 and the lower surface of the end closure. Furthermore the heating elements 41 are suspended in the upper end closure 36. In the lower part of the tube 34 is a ring 46 projecting permanently into the tube. The lower end closure 37 is provided with a bracket 47 and a guide 48 and is vertically slidable and turnable on a guide 49. Lowering and raising is done with the help of an operating cylinder 50 attached on the stand, the operating rod 51 of which is connected to the guide. The pressure furnace unit also comprises a movable press stand to take up the com pressive forces operating on the end closures. Also this press stand is of the type containing two yokes 53 and 54, two spacers 55 and a strip sheath 56 holding them together. The stand is provided with brackets 57 to journal wheels 58 running on rails 59. On the lower end closure is a cylinder 60 of insulating material. On this stands a billet 61. During the pressure sintering the stand 52 is moved in above the high-pressure chamber, during emptying and charging the stand is a little dis tance away from the highpressure chamber so that the lower end closure can be lowered and turned as can be seen in FIGS. 1 and 5.

Large containers require a long heating time. A container having an outer diameter of 350 mm with a highspeed steel powder which has been cold compressed at high pressure piror to heating requires as a rule a heating time of more than 8 hours, with the method used up to now, before the powder in the center has reached such a high temperature, 1100C, that the powder is bonded together and the desired density is obtained during cold compression. However, if the container contains a harmless gas which fills up the spaces between the powder grains, a quicker heat transfer is obtained, by means of convection inside the powder, from the exterior part of the container to its interior part. In many cases the time of heating may be reduced by more than 50%.

We claim:

I. In a method of manufacturing high-speed steel from metal powder which comprises enclosing powder or a green body shaped of powder in a container, heating the container with its contents and subjecting the container to a high, all sided fluid pressure in a pressure furnace at such a temperature that the powder body is bonded together and a solid body is formed, the steps of degassing the container and powder therein and, after degassing, supplying to the container gas consisting essentially of nitrogen, said gas acting during the subsequent heating as a heat transfer agent circulating within the container, heating the container with the gas therein, and subjecting the closed, heated container to a pressure which is required for bonding at a selected pressing temperature, said nitrogen being included as a constituent in the manufactured material. 

1. IN A METHOD OF MANUFACTURING HIGH-SPEED STEEL FROM METAL POWDER WHICH COMPRISES ENCLOSING POWDER OR A GREEN BODY SHAPED OF POWDER IN A CONTAINER, HEATING THE CONTAINER WITH ITS CONTENTS AND SUBJECTING THE CONTAINER TO A HIGH, ALL SIDED FLUID PESSURE IN A PRESSURE FURNACE AT SUCH A TEMPERATURETURE THAT THE POWDER BODY IS BONDED TOGETHER AND A SOLIE BODY IS FORMED, THE STEPS OF DEGASSING THE CONTAINER AND POWDER THEREIN AND, AFTER DEGASSING, SUPPLYING TO THE CONTAINER GAS CONSISTING ESSENTIALLY OF NITROGEN, SAID GAS ACTING DURING THE SUBSEQUENT HEATING AS A HEAT TRANSFER AGENT CIRCULATING WITHIN THE CONTAINER, HEATING THE CONTAINER WITH THE GAS THEREIN, AND SUBJECTING THE CLOSED, HEATED CONTAINER TO A PRESSURE WHICH IS REQUIRED FOR BONDING AT A SELECTED PRESSING TEMPRATURE, SAID NITROGEN BEING INCLUDED AS A CONSTITUENT IN THE MANUFACTURED MATERIAL. 